Field of the Invention
This invention relates to a method for the manufacture of alkali metal chromates from chrome ores by a lime-free double roast wherein the extraction of alumina into the leach liquor is essentially eliminated by controlling a number of process variables within certain critical limits.
2. Description of the Prior Art
It is well known that alkali metal chromates may be produced by roasting of chrome ore, soda ash and lime in an oxygen atmosphere. The roasting operation may be carried out either as a single or double roast at temperatures of around 1150.degree. C., either in a rotary kiln or in a rotary hearth furnace. In a single roast operation, the roast leaving the kiln is cooled, leached, and washed to recover the soluble alkali metal chromate product. The residue left after leaching is either discarded or a part is recycled back to the kiln. In the double roast operations, all the residue from the first roast is recycled back to the process. A part of the residue may be recycled to the first roast as a diluent and the remaining part may be subjected to a second roast in the presence of additional soda ash. The roast leaving the second roast operation may then be cooled, leached and washed to recover soluble alkali metal chromate. The residue left behind is either discarded or a part recycled back to the process. Processes of this type are disclosed in prior art publications with many modifications.
In one modification, lime has been added to the mix to insolubilize and fix the alumina present as calcium aluminate compounds. The addition of the lime however, also forms calcium chromate compounds and these compounds are responsible for pollution and hygiene problems associated therewith. The lime is added to the roast mix to prevent extraction of alumina into the leached liquor in order to avoid the necessity of removing the alumina from the product liquor in recovery of the product. Removal of alumina from the product liquor is quite difficult because of the formation of gelatinous precipitates which are difficult to filter.
The prior art discloses processes conducted in the absence of added lime as well as in the presence of added lime. For example, U.S. Pat. No. 797,323, a 1905 patent, discloses a double roast, lime-free process using a relatively high percentage of soda ash but no residue is recycled. The amount of soda ash is controlled to avoid fusion of the mass during roasting but, this system is not applicable to rotary kiln operations. Furthermore, excess alumina is extracted into the product liquors. A similar process is described in British Pat. No. 270,143 wherein a lime-free roast is disclosed in the first step, and wherein the roast mix contains only a mixture of ore and soda ash with the soda ash being maintained in generally high amounts and with lime used in the second step.
A process is disclosed in U.S. Pat. No. 1,948,143 wherein a mixture of lime, soda ash, and leached residue from a preceding first stage roast, are used to roast the chrome ore. U.S. Pat. No. 2,199,929 discloses a continuous countercurrent leaching step in a lime-free double roast but it describes no procedure to prevent extraction of alumina into the leach liquor. Also, U.S. Pat. No. 1,752,863 discloses a method for the roasting of chromium ores in the presence of soda ash and lime with subsequent leaching of the products but wherein a portion of the leaching residue may be used to replace some of the lime in the initial step. A similar process may be found in U.S. Pat. No. 3,812,234 wherein recycle treatment residue is considered to be an equivalent diluent to dolomite, iron ore and the like, in the roasting of chromium ores with soda ash, but in this patent, the chromium-containing material is initially heated under fluidizing conditions with specific preheating and fluidizing procedures required for good recovery of products.
U.S. Pat. No. 3,819,800 discloses a lime-containing roasting process wherein a small amount of lime in the range of 1-3% is maintained in the roast mix and alumina is extracted together with alkali metal chromate in the product liquor with provisions made for removal of the alumina from the product liquor. U.S. Pat. No. 1,631,170 and British Pat. No. 1,057,678 disclose processes wherein lime-free roasting of chromium ores is carried out in the presence of soda ash and wherein leaching of the resulting product is carried out under acidic conditions. Sodium aluminate is produced in both processes and complicated separation steps are required to prevent extraction of alumina into the product chromate solutions.
All of these prior art processes indicate that various techniques have been used so as to conduct the roasting operation under conditions to minimize material costs and still recover good yields of the desired alkali metal chromate. The process of the present invention meets these objectives.